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A Remark on the Krull-Schmidt-Azumaya Theorem

Published online by Cambridge University Press:  20 November 2018

B. L. Osofsky*
Affiliation:
Rutgers University, New Brunswick, New Jersey
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It is well known that if a module M is expressible as a direct sum of modules with local endomorphism rings, then such a decomposition is essentially unique. That is, if M = ⊕iIMi = ⊕jJNj then there is a bijection f: I → J such that Mi is isomorphic to Nf(i) for all iI (see [1]). On the other hand, a nonprincipal ideal in a Dedekind domain provides an example where such a theorem fails in the absence of the local hypothesis. Group algebras of certain groups over rings R of algebraic integers is another such example, where even the rank as R-modules of indecomposable summands of a module is not uniquely determined (see [2]). Both of these examples yield modules which are expressible as direct sums of two indecomposable modules in distinct ways. In this note we construct a family of rings which show that the number of summands in a representation of a module M as a direct sum of indecomposable modules is also not unique unless one has additional hypotheses.

Type
Research Article
Copyright
Copyright © Canadian Mathematical Society 1970

References

1. Azumaya, G., Corrections and supplementaries to my paper concerning Krull-Remak-Schmidt's theorem, Nagoya Math. J. 1 (1950), 117-124.Google Scholar
2. Reiner, I., Failure of the Krull-Schmidt theorem for integral representations, Michigan Math. J. 9 (1962), 225-231.Google Scholar